In a mobile communication system, Device-to-Device (D2D) communication refers to such a mode in which adjacent terminals can transmit data to each other over a direct link in a short-distance range, where neither the data are required to be forwarded by a central node (i.e., a base station), nor information is required to be transmitted between User Equipments (UEs) over a traditional cellular link. Communication in a D2D proximity service is typically conducted under the control by or with the aid of the network side, where an evolved Node B (eNB) may even allocate dynamically a resource for a UE communicating in the D2D proximity service. Mechanisms of the D2D proximity service include the Device to Device (D2D) discovery mechanism and the D2D communication mechanism. With the D2D discovery mechanism, a terminal discovers an adjacent terminal; and with the D2D communication mechanism, two UEs proximate to each other can have a link set up directly between the two UEs.
Further to the D2D discovery and D2D communication technologies, the UE can transmit data with the network or the other UE by selecting a relay UE. There are types of relays in the following two scenarios in the D2D system:
(1) A UE-to-network relay: an out of coverage source UE going to communicate with the network can select an adjacent in-coverage UE for the source UE to repeat data for the source UE, so that the out of coverage UE communicates with the network through the in-coverage UE, where the in-coverage UE is referred to a UE-to-network relay. As illustrated in FIG. 1, a UE2 which is not covered by the network but going to communicate with the network can select a UE1 as a relay node, so that uplink and downlink signals of the UE2 is forwarded by the UE1, where communication between the UE1 and the UE2 is embodied as D2D communication, and communication between the UE1 and the network is embodied as cellular communication.
(2) A UE-to-UE relay: if there is such a distance between a source UE and a destination UE that no D2D discovery or D2D communication can be made directly between them, then a third-party UE adjacent to both the source UE and the destination UE will be selected as a relay node responsible for repeating data, so that a D2D discovery or D2D communication is made between these two UEs, where the third-party UE is referred to a UE-to-UE relay. As illustrated in FIG. 2, the UE2 can have its information forwarded by the UE1 in order to communicate with a UE4 which is not located in a range of direct communication with the UE2, where data are transmitted between the UE1 and the UE2, and between the UE1 and the UE4 through D2D communication.
In the communication process, all of the source UE, the destination UE, and the relay UE are mobile, so such a problem will be faced by the source UE operating with either the UE-to-network relay or the UE-to-UE relay that the source UE needs to reselect the relay UE capable of providing the source UE and the destination UE with the data transmission service, but there has been absent so far a corresponding solution to this problem.